This invention relates to a background compensating type plutonium air monitor and, more particularly, to a plutonium air monitor in which the background counting rate of a plutonium measuring channel is erased digitally by using relationship in measuring the permissible concentration of the plutonium wherein the background counting rate of a plutonium measuring channel which is caused by the partial transition of .alpha.-ray energy spectrum of the daughter nuclides of the radon and the thoron to low energy side has relationship with the counting rate due to the daughter nuclide of the radon and the thoron which have energy exceeding 5.5 MeV.
The air pollution due to the plutonium dust is extremely harmful to the human body and, accordingly, the maximum permissible concentration of the plutonium dust in a working environment is legally determined at an extreme low concentration, such as 2.times.10.sup.-12 .mu.Ci/cm.sup.3. Daughter nuclides of natural radon and thoron ordinarily exist in the atmosphere making the substantial part the radon and its daughter nuclide. The concentration of the radioactivity of the daughter nuclides depends on the condition of the atmosphere and varies in a range from 10.sup.-9 to 10.sup.-12 .mu.Ci/cm.sup.3 to emit .alpha.-rays as the plutonium does. Therefore, when the permissible concentration of the plutonium is measured by a fixed filterpaper type dust monitor under a condition where the daughter nuclides stated above exist, the background affection due to natural radionuclides such as the radon and the thoron daughter must be removed.
The .alpha.-ray energy emitted from the daughter nuclide of the radon and the thoron which has 6.about.9 MeV is fortunately different from that emitted from the plutonium which has 5.2 MeV. Therefore, a plutonium air monitor having relatively less background counting can be obtained if the permissible concentration of the plutonium is measured by setting the plutonium measuring channel in the neighbourhood of 5.2 MeV by means of a detector having better energy resolving power and a single channel analyzer. However, even with the measurement stated above, some background counting remains in the plutonium measuring channel due to the transition of the .alpha.-ray spectrum of the daughter nuclides of the radon and the thoron which are collected on the filter-paper and, further, the background counting varies with the variation in the concentration of the daughter nuclides of the radon and the thoron to affect remarkably the measurement of the low-concentration plutonium.
The fundamental circuit of a background compensating type plutonium air monitor of the prior art is shown in FIG. 1. In the FIGURE, the radio dust collected in a filter-paper 1 is converted into electrical signals by means of a detector 2 and the signals are amplified by means of an amplifier 3. The output from amplifier 3 is applied to a single channel analyzer which is comprised of an upper-level discriminator 4, a lower-lower level discriminator 5 and an anti-coincidence gate circuit 6. According to the background compensating system of the plutonium air monitor of the prior art, the counting rate of the daughter nuclide of the radon and the thoron exceeding 5.5 MeV coming from upper-level discriminator 4 is applied to a linear-rate meter f and the counting rate of the plutonium measuring channel coming from lower-level discriminator 5 and anti-coincidence gate circuit 6 is applied to a linear-rate meter g. Thus, random pulse signals are once applied to the linear-rate meter to be converted to a direct current output and, thereafter, a weight corresponding to the reciprocal of a relation function a is applied to the output of linear-rate meter f by means of a variable resistor h to obtain a direct current output equivalent to the background of the plutonium measuring channel. Finally, the output from variable resistor h and the output from linear-rate meter g are applied to a differential amplifier i to erase the background of the plutonium measuring channel.
The difficulty of the background compensation system of the prior art is in that the change-over of the measuring ranges is highly troublesome due to the simultaneous changeover of two sets of linear-rate meters and in that the adjustment of the whole system is not easy due to the long time-constant of the linear-rate meters. Therefore, it is desirable that the permissible concentration can be measured by means of a log-rate meter when the radiation is controlled by means of an air monitor. A system is indeed found in the market in which no change-over of the measuring range is needed in the compensation system of the prior art but the result is given in that the detecting sensitivity and the indicating precision become worse and an ability is insufficiently obtained in order to control the air pollution.
As stated above, the background compensating technique of the plutonium air monitor of the prior art depends on a linear-rate meter and a differential amplifier and uses a number of parts so that there remain difficulties in the measuring range change-over and control and the detecting sensitivity.